Method for the stabilization of a fluidized bed in a roasting furnace

ABSTRACT

This invention relates to a method for stabilizing a fluidized bed used in roasting by adjusting the oxygen content of the roasting gas in the bed. The fine-grained material for roasting is fed into the furnace above the fluidized bed and the roasting gas, which causes the fluidizing, is fed into the bottom of the furnace through a grate. In this method, the total amount of oxygen in the roasting gas to be fed and the average total oxygen requirement of the material to be roasted are calculated and the ratio between them regulated so that the oxygen coefficient in the bed is over 1.

[0001] This invention relates to a method for stabilizing a fluidizedbed used in roasting by adjusting the oxygen content of the roasting gasin the bed. The fine-grained material for roasting is fed into thefurnace above the fluidized bed and the roasting gas, which causes thefluidized bed, is fed into the bottom of the furnace through a grate. Inthis method, the total amount of oxygen in the roasting gas to be fedand the average total oxygen requirement of the material to be roastedare calculated and the ratio between them regulated so that the oxygencoefficient in the bed is over 1.

[0002] Roasting can be done in several different furnaces. Nowadayshowever, the roasting of fine-grained material usually takes place withthe fluidized bed method. The material to be roasted is fed into theroasting furnace via the feed units in the wall of the furnace above thefluidized bed. On the bottom of the furnace there is a grate, via whichoxygen-containing gas is fed in order to fluidize the concentrate. Theoxygen-containing gas usually used is air. There are usually in theorder of 100 gas nozzles/m² under the grate. As the concentrate becomesfluidized, the height of the feed bed rises to about half that of thefixed material bed. The pressure drop in the furnace is formed by theresistance of the grate and that of the bed. The resistance of the bedis more or less the mass of the bed when the bed is in a fluidizedstate. The pressure drop is in the range of 240-280 mbar.

[0003] The roasting of sulfides is described for example in the book byRosenqvist, T.: Principles of Extractive Metallurgy, pp. 245-255,McGraw-Hill, 1974, USA. According to Rosenqvist, roasting is theoxidizing of metal sulfides, giving rise to metal oxides and sulfurdioxide. For example, zinc sulfide and pyrite oxidize as follows:

2ZnS+3O₂→2ZnO+2SO₂  (1)

2FeS₂+5½O₂→Fe₂O₃+4SO₂  (2)

[0004] In addition, other reactions may occur such as the formation ofSO₃, the sulfating of metals and the formation of complex oxides such aszinc ferrite (ZnFe₂O₄). Typical materials for roasting are copper, zincand lead sulfides. Roasting commonly takes place at temperatures belowthe melting point of sulfides and oxides, generally below 900-1000° C.On the other hand, in order for the reactions to occur at a reasonablerate, the temperature must be at least of the order of 500-600° C. Thebook presents balance drawings, which show the conditions demanded forthe formation of various roasting products. For instance, when air isused as the roasting gas, the partial pressure of SO₂ and O₂ is about0.2 atm. Roasting reactions are strongly exothermic, and therefore thebed needs a cooling arrangement.

[0005] The calcine is removed from the furnace partially via an overflowaperture, and is partially transported with the gases to the waste heatboiler and from there on to the cyclone and electrostatic precipitators,from where the calcine is recovered. Usually the overflow aperture islocated on the opposite side of the furnace from the feed units. Theremoved calcine is cooled and ground finely for leaching.

[0006] For good roasting it is important to control the bed i.e. the bedhas to be of stable construction and have other good fluidizingproperties and the fluidizing has to be under control. Combustion shouldbe as complete as possible, i.e. the sulfides must be oxidizedcompletely into oxides. The calcine has also to come out of the furnacewell, i.e. the particle size of the calcine must be within certainlimits. The particle size of the calcine is known to be affected by thechemical composition and mineralogy of the concentrate as well as by thetemperature of the roasting gas.

[0007] Zinc sulfide concentrates handled in zinc roasters have becomemore impure over the course of time. Concentrates are no longer anywherenear pure zinc blende, sphalerite, but may contain a considerable amountof iron. Iron is either dissolved in the sphalerite lattice or in theform of pyrite or pyrrhotite. In addition, concentrates often containsulfidic lead and/or copper. The chemical composition and mineralogy ofthe concentrates vary enormously. In this way the amount of oxygenrequired for oxidation of the concentrates also varies, as does theamount of heat produced on combustion. In the technique currently in usethe roaster concentrate feed is regulated according to the temperatureof the bed using fuzzy logic for example. Thus there is a danger thatthe oxygen pressure in the fluidized bed drops too low i.e. that theamount of oxygen is insufficient to roast the concentrate. As a result,the bed does not agglomerate normally but remains too fine and at thesame time the back pressure of the bed may fall too low, because a finebed stops fluidizing and channeling occurs. The real oxygen demand of afluidized bed is unknown, because generally the concentrate mix is notcalculated continuously in advance on the basis of its precisecomposition, nor are there any devices in the bed for measuring theoxygen content. Therefore the operation of a fluidized bed furnace isdifficult to regulate and keep stable.

[0008] The particle size of the zinc sulfide concentrates to be treatedalso varies. As a result, it is difficult to know which part of theconcentrate will burn in the bed when and which part above the bedtransported by the exhaust gas. If a significant amount of thecombustion occurs above the bed, less energy is created in the bed thanusual and, depending on the regulation method, this may increase thefeed.

[0009] As stated above, it is known from balance calculations andbalance diagrams in the literature that copper and iron together andseparately form oxysulfides, which are molten at roasting temperaturesand even lower temperatures too. Similarly, zinc and lead as well asiron and lead both form sulfides molten at low temperatures. This kindof sulfide appearance is possible and the likelihood grows if the amountof oxygen in the bed is smaller than that normally required to oxidizethe concentrate.

[0010] During fluidized bed roasting agglomeration of the productnormally occurs, i.e. the calcine is clearly coarser than theconcentrate feed. The above-mentioned formation of molten sulfidesnevertheless increases agglomeration to a disturbing degree, in that theagglomerates with their sulfide nuclei remain moving around the grate.Agglomerates cause build-ups on the grate and, over the course of time,block the gas nozzles under the grate. It has been noticed in zincroasters that build-ups containing impure components are formed in thefurnace particularly in the part of the grate under the concentrate feedunits.

[0011] In the article by Nyberg, J. et al: Recent Process Improvementsin the Kokkola Zinc Roaster, Lead-Zinc Symposium 2000, Pittsburgh, USA,Oct. 22-25, 2000, pages 399-415, it is stated that the roaster fluidizedbed generally moves towards an unstable state when the percentage of thefinest fraction in the bed increases. In this case the temperatures ofthe control thermo-elements diverge, as a result of the fact that thebed is too fine for fluidization and that channeling occurs. Inaddition, the back pressure of the bed drops and the feed drops.

[0012] The literature contains research on a zinc sulfide oxidationmodel, which works at extremely low oxygen contents. According to thismodel, zinc oxide is formed at low oxygen pressures through gasreactions and not through a solid-gas reaction as normal. This meansthat condensed zinc oxide is extremely fine. However, the power of thefans below the grate is not always sufficient to increase gas feed andlikewise the amount of oxygen. On the other hand, the acid plant afterthe roaster may also cause capacity limitations. The concentrate mayalso be so fine, that if the gas feed is increased, the material will nolonger stay in the fluidized bed but instead will fly out in the flow ofgas. Sometimes the quality of the concentrate does not allow changes inthe temperature of the bed and with it the reduction in feed and by thismeans the increase in the amount of oxygen to a sufficient level. Theremay also be situations where neither of the above regulating methods ispossible.

[0013] Different ways of regulating roasting conditions have beenattempted. U.S. Pat. No. 5,803,949 relates to a method of stabilizingthe fluidized bed in the roasting of metal sulfides, where stabilizingoccurs by controlling the particle size of the feed. In U.S. Pat. No.3,957,484 stabilization occurs by feeding the concentrate as a slurry.In the article MacLagan, C. et al: Oxygen Enrichment of Fluo-SolidsRoasting at Zincor, Lead-Zinc Symposium 2000, Pittsburgh, USA, Oct.22-25, 2000, pages 417426, it is stated that the oxygen content of theroaster exhaust gas is controlled by measurements taken from the gasline after the boiler or the cyclone. These measurements do not,however, tell of the status of the fluidized bed, because the gas linemeasurements already include leakage air.

[0014] In order to correct the deficiencies presented above, a methodaccording to the present invention has now been developed to stabilize afluidized bed for use in roasting fine material by regulating the oxygencontent of the gas in the bed. In order that for instance zinc sulfideconcentrate be oxidized into zinc oxide, the oxygen coefficient of thefluidized bed should in theory be at least one. The oxygen coefficientis obtained when the total oxygen feed of the roasting gas is calculatedand compared to the total oxygen requirement of the concentrate feedmixture. According to the method now developed, the oxygen coefficientis adjusted to be over 1, preferably at least 1.03. In order to effect amore accurate adjustment, the oxygen content is also measured in the beditself. The stabilization of the fluidized bed by regulating the oxygencoefficient prevents capacity losses, which result from the build-upformed on the grate and the production stoppages they cause. Theessential features of the invention will be made apparent in theattached claims.

[0015] According to the present method, it is possible to do theadjustment of the oxygen coefficient on the basis of two process data:first calculate the average oxygen requirement of the feed mixture(NM³O₂/t concentrate mixture) using the calculated oxygen requirementsof the studied chemical and mineralogical composition of the eachconcentrate. The oxygen requirement of the concentrate mixture isentered into the process control equipment whenever the mixture ischanged. The second process data required is the total oxygenrequirement, which is calculated on the basis of the oxygen requirementof the feed mixture and the concentrate feed (t/h) to be measuredcontinuously. During roasting, the process control equipment measuresthe oxygen coefficient of the process i.e. it compares the total oxygenfeed to the calculated total oxygen requirement. The total oxygen feedis obtained by measuring the amount of gas to be fed via the grate andits oxygen content. The control equipment is given appropriate limitvalue, and if the oxygen coefficient falls below this limit, theequipment reacts in the prescribed manner e.g. with an alarm or acertain adjustment procedure. These kinds of adjustment procedures are,depending on the situation, the adjustment of the oxygen coefficient tothe right range, either by changing the temperature, the amount of grateair or oxygen enrichment either separately or together in differentcombinations. Pure oxygen may be fed with the grate gas as oxygenenrichment.

[0016] As stated previously, with embodiments of the prior art ofroasting it has not been able to determine which part of the concentratewill be oxidized in the bed and which part only above the bed and whatthe percentage of leakage air will be. Thus there is no precise pictureof the sufficiency of the amount of oxygen in the bed. Therefore, inorder to specify the adjustment action, it is necessary to carry outoxygen content measurement in the bed also. In the present invention thefine-adjustment of oxygen content can be done either continuously or forexample only when changing the feed mixture. Probes for instance areused as the measurement device. On the basis of this measurement, theactions described above are carried out as required in order to adjustthe oxygen coefficient to the right range. In particular when usingoxygen enrichment the avoidance of wasted costs should be kept in mindor feeding oxygen in excess, since pure oxygen is expensive.

[0017] The invention is described further in the following example:

EXAMPLE 1

[0018] A concentrate with a sphalerite composition was compared to azinc concentrate containing pyrite. Calculating the oxygen requirementof the concentrates showed that the oxygen requirement of the sphaleriteconcentrate in roasting is 338 Nm³/t and for the pyrite-containingconcentrate 378 Nm³/t, in other words the oxygen requirement of thepyrite-containing concentrate is over 10% greater than that of thesphalerite concentrate. The mineral contents of the concentrates areshown in Table 1. TABLE 1 Pyrite-containing Sphalerite concentrateconcentrate Mineral w-% w-% CuFeS₂ 0.09 1.73 FeS 2.54 2.85 FeS₂ 0.3521.63 ZnS 91.66 68.11 PbS 1 3.11 CdS 0.24 0.18 SiO₂ 0.94 0.43 CaSO₄ 0.830.1 CaCO₃ 1.05 0.5 others 1.3 1.36

1. A method of stabilizing a fluidized bed used in roasting of afine-grained material, characterized in that the total amount of oxygenin the roasting gas to be fed and the average total oxygen requirementof the material to be roasted are calculated and the ratio between themregulated so that the oxygen coefficient in the bed is over 1, and inorder to adjust the oxygen coefficient, oxygen content measurement istaken from the fluidized bed.
 2. A method according to claim 1,characterized in that the oxygen coefficient is adjusted to be at least1.03.
 3. A method according to claim 1, characterized in that the oxygencoefficient is adjusted by changing the temperature.
 4. A methodaccording to claim 1, characterized in that the oxygen coefficient isadjusted by changing the amount of roasting air.
 5. A method accordingto claim 1, characterized in that the roasting gas is air.
 6. A methodaccording to claim 1, characterized in that oxygen-enriched air is usedas the roasting gas.
 7. A method according to claim 6, characterized inthat the oxygen coefficient is adjusted by changing the oxygenenrichment of the roasting gas.
 8. A method according to claim 1,characterized in that oxygen content measurement from the bed is madecontinuously.
 9. A method according to claim 1, characterized in thatoxygen content measurement from the bed is carried out when changing thefeed mixture.
 10. A method according to claim 1, characterized in thatthe material to be roasted is a zinc concentrate.
 11. A method accordingto claim 1, characterized in that the material to be roasted is aniron-containing sulfide concentrate.